Koichi was trained as a Master framebuilder at the legendary 3Rensho company in Japan, building bicycles for professional Keirin riders (notably, the ten-time Sprint World Champion Koichi Nakano).

In 1987, Koichi came to the United States and founded his own bicycle building company, Yamaguchi Bicycles, based in Rifle, Colorado. He began working closely with the US Cycling Federation as the National Team Mechanic in 1988 and, in 1989, his projects included prototype frames for the US National Cycling Team. That same year, Koichi became the official framebuilder for the United States Olympic Cycling Team.